Praise be to Allah,
Sufyaan ibn ‘Uyaynah narrated to us from [Muhammad ibn Shihaab] az-Zuhree from Anas ibn Maalik who said,
“The Prophet fell off a horse and suffered some scratches on his right side. So we went to visit him. When the time for prayer came, he led us sitting down and we also prayed sitting down. When he concluded the prayer he said [to us], ‘The imaam is only appointed to be followed. Therefore, when he says Allaahu Akbar, say, Allahu Akbar; and when he bows, then bow.’ (Sufyaan said on one occasion, ‘and when he prostrates then prostrate. ‘) When he says, Sami ‘allaahu timan hamidah (Allah hears those who praise Him), then say, Rabbanaa wa lak-al-hamd (All praise belongs to You, our Lord); and if he prays sitting down [for an acceptable reason] then pray sitting down as well. “‘
(Musnad Ahmad, Vol. 3, p. 110. The isnaad of this hadeeth is Saheeh according to the conditions set by al-Bukhaaree and Muslim.)
Source:
Thulaathiyyaat Musnad Ahmad ibn Hanbal (may Allah have mercy on him)
Reports of Imaam Ahmad that have only three narrators between him and the Prophet, peace, and blessings be upon him.
Shaykh Muhammad ibn Ahmad ibn Saalim as–Saffaareenee al–Hanbalee ~(1114 –1188 AH)