Anas bin Sirin narrated: We were coming from Kufah with Sayyiduna Anas bin Malik (may Allah be pleased with him), until, when we reached Atit, the place was one where there was mud. So Sayyiduna Anas (may Allah be pleased with him) offered the obligatory prayer while on his mount, and then he said: "Before today I have never offered an obligatory prayer on a mount."
Imam al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2982