Mishkat al-Masabih 1312
Zaid b. Arqam, on seeing some people praying in the forenoon, said:
They well know that prayer at another time than this is more excellent. God’s Messenger said, “The prayer of those who are penitent is observed when the young weaned camels feel the heat of the sun.”* 

* Young camels are said to feel the sand too hot when a quarter of the day has passed.

Muslim transmitted it.
وَعَنْ زَيْدِ بْنِ أَرْقَمَ أَنَّهُ رَأَى قَوْمًا يُصَلُّونَ مِنَ الضُّحَى فَقَالَ: لَقَدْ عَلِمُوا أَنَّ الصَّلَاةَ فِي غَيْرِ هَذِهِ السَّاعَةِ أَفْضَلُ إِنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «صَلَاةُ الْأَوَّابِينَ حِينَ تَرْمَضُ الْفِصَالُ» . رَوَاهُ مُسْلِمٌ