بُلُوغُ الْمَرَامِ
Bulugh al-Maram 395
The Book of Prayer
وَعَنْ زَيْدِ بْنِ أَرْقَمَ; أَنَّ رَسُولَ اَللَّهِ - صلى الله عليه وسلم -قَالَ: { صَلَاةُ اَلْأَوَّابِينَ حِينَ تَرْمَضُ اَلْفِصَالُ } رَوَاهُ اَلتِّرْمِذِيُّ . [1] .
Narrated Zaid bin Arqam (RA): Allah's Messenger (ﷺ) said: "The prayer of those who are penitent is offered when the young weaned camels feel the heat of the sand." [Reported by at-Tirmidhi].
Sunnah.com reference: Book 2, Hadith 299