In the Hadiths narrated from Shiite Imams, Shiite people have been strongly advised to say prayers together with non-Shiite Muslims in Masjid al-Ḥarām and Masjid an-Nabawi, to participate in Sunni Muslims’ prayers, and to follow their lead in saying prayers; these are the Hadiths narrated by Shiite Imams; this is solidarity, this is fostering solidarity among Muslim brothers.
Iranian Supreme Leader, Grand Ayatollah Khamenei considers following the lead of a Sunni prayer leader as permissible and says: in the Hadiths narrated from Shiite Imams, Shiite people have been strongly advised to say prayers together with non-Shiite Muslims in Masjid al-Ḥarām and Masjid an-Nabawi, to participate in Sunni Muslims’ prayers, and to follow their lead in saying prayers; these are the Hadiths narrated by Shiite Imams; this is solidarity, this is fostering solidarity among Muslim brothers.
According to Ijtihad Network, the fatwas of Shiite Grand Ayatollahs regarding following the lead of Sunni prayer leaders in Masjid al-Ḥarām and Masjid an-Nabawi are presented as follows:
The late Imam Khomeini says: when Salat al-jama’ah (Congregational Prayer) is either held in Masjid al-Ḥarām or Masjid an-Nabawi, the believers are not allowed to leave those holy places or to avoid participating in the Salat, but they should say their prayers in jama’ah together with other Muslims.
Ayatollah Golpayegani: If inevitable, following the lead of Sunni prayer leaders is allowed, and in normal conditions, participating in Salat al-jama’ah held by Sunni people is advisable, but the Salat should be personally repeated if possible.
Ayatollah Bahjat: When Salat al-jama’ah (Congregational Prayer) is either held in Masjid al-Ḥarām or Masjid an-Nabawi, Shiite Muslims are advised not to leave there if they have nothing necessary to do outside, and they have better participate in the Salat.
Ayatollah Safi: If inevitable, following the lead of Sunni prayer leaders in saying prayers is permitted. However, under normal circumstances, it is advisable to participate in Salat al-jama’ah held by Sunni Muslims if it promotes solidarity with them and drops false charges against the Shiites, and there is no need to repeat the Salat personally though it is recommended to be done. Moreover, leaving Masjid al-Ḥarām or Masjid an-Nabawi when Salat al-jama’ah is being held is not advisable but permissible if necessary.
Ayatollah Khoie and Ayatollah Tabrizi: Participating in this Salat is allowed if Fatihah al-kibtab and the Surah are individually recited in silence. Moreover, if Friday prayer is to be participated, the Midday prayer should be repeated personally when the Friday prayer is finished.
Ayatollah Sistani: Following the lead of Sunni prayer leaders in Salat al-jama’ah is permissible, but the Ma’mum (the person being led by an Imam in Prayer) should recite Al-Fatihah and the Surah individually in silence if possible, otherwise he/she should recited them in his/her mind without verbalizing them loudly, and if the Friday prayer is being held, the Midday prayer should be repeated individually when the Salat is finished.
Ayatollah Shobeiri Zanjani: Dear Hajjis (Pilgrims to Mecca) are required to seriously avoid doing anything which may tarnish the reputation of Shiism, and it is highly recommended that they participate in Salat al-jama’ah held by Sunni Muslims, though they should repeat their prayers individually under the current circumstances (Anthology of the Haj rituals according to the fatwas of Grand Ayatollah Khomeini along with the annotations written by a select number of Maraji (Grand Ayatollahs), Tehran, Mash’ar Publications, 1380 S.H., p.287).
Grand Ayatollah Khamenei also allows following the lead of Sunni prayer leaders in Salat al-jama’ah (collections of juridical edicts, Tehran, Al-Hoda Publications, 1381 S.H., P.539, and P.197)
Where are the sources for these hadiths? Why not include the actual hadiths if there are so many narrated by Ahlul Bayt (as)?