Muwatta Malik 355

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، قَالَ رَأَيْتُ أَنَسَ بْنَ مَالِكٍ فِي السَّفَرِ وَهُوَ يُصَلِّي عَلَى حِمَارٍ وَهُوَ مُتَوَجِّهٌ إِلَى غَيْرِ الْقِبْلَةِ يَرْكَعُ وَيَسْجُدُ إِيمَاءً مِنْ غَيْرِ أَنْ يَضَعَ وَجْهَهُ عَلَى شَىْءٍ ‏.‏

Yahya related to me from Malik that Yahya ibn Said said, "I saw Anas ibn Malik on a journey praying on a donkey facing away from the qibla. He did the raka and the sajda by motioning with his head, without putting his face on anything

Reference

Muwatta Malik 355

In-book

Book 9, Hadith 30