Sahih Muslim 748
Sahih Muslim · Book 6: The Book of Prayer - Travellers · Salat al-Awwabin (the prayer of the penitent) is when the young camels feel the heat of the hot sand
Zaid b. Arqam reported that the Messenger of Allah (ﷺ) went out to the people of Quba' and saw them observing prayer; upon this he said:
حَدَّثَنَا زُهَيْرُ بْنُ حَرْبٍ ، حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ ، عَنْ هِشَامِ بْنِ أَبِي عَبْدِ اللَّهِ ، قَالَ حَدَّثَنَا الْقَاسِمُ الشَّيْبَانِيُّ ، عَنْ زَيْدِ بْنِ أَرْقَمَ ، قَالَ خَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم عَلَى أَهْلِ قُبَاءٍ وَهُمْ يُصَلُّونَ فَقَالَ " صَلاَةُ الأَوَّابِينَ إِذَا رَمِضَتِ الْفِصَالُ " .
The prayer of the penitent should be observed when the young weaned camels feel heat of the sun.