Ibrahim bin Ubayd bin Rifa'ah narrates from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both): I went to Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) and found him leading his companions in the Asr prayer while sitting, and they were also sitting. Ibrahim said: I kept watching him until he gave the salam, then I said to him: 'Ghafara Allahu lak' (May Allah forgive you) - you are a Companion of the Messenger of Allah (peace and blessings be upon him), and you lead them in prayer while sitting?
He replied: "I am ill, so I am praying sitting, and it was I who ordered them to sit as well, for I heard the Messenger of Allah (peace and blessings be upon him) saying: 'The imam is a shield (and a leader), so when he prays standing, you too should pray standing, and when he prays sitting, all of you should pray sitting.'"
The chain of this hadith contains Khalid bin Ilyas, who is matruk al-hadith (abandoned); however, praying sitting behind the imam is also mentioned in other authentic hadith.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب الصلاة / 413
Hadith Gradingمحدثین:إسناده ضعيف جدًا، موقوف
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :موقوف، إسناده ضعيف جدًا. » «أخرجه الدارقطني فى «سننه» برقم: 1564، وعبد بن حميد فى "المنتخب من مسنده"، 1152، وأورده ابن حجر فى "المطالب العالية"، 413، 413، 630»