بُلُوغُ الْمَرَامِ
Bulugh al-Maram 428
The Book of Prayer
وَعَنْ عَلِيٍّ - رضى الله عنه - قَالَ: قَالَ اَلنَّبِيُّ - صلى الله عليه وسلم -{ إِذَا أَتَى أَحَدُكُمْ اَلصَّلَاةَ وَالْإِمَامُ عَلَى حَالٍ, فَلْيَصْنَعْ كَمَا يَصْنَعُ اَلْإِمَامُ } رَوَاهُ اَلتِّرْمِذِيُّ بِإِسْنَادٍ ضَعِيفٍ . [1] .
Narrated 'Ali bin Abi Talib (RA): Allah's Messenger (ﷺ) said: "If one of you comes to Salat (prayer) and the Imam is at a certain position, he must do as the Imam is doing." [Reported by at-Tirmidhi with a weak chain of narrators].
Sunnah.com reference: Book 2, Hadith 335