Jami` at-Tirmidhi 411

Amr bin Uthman bin Ya'la bin Murrah narrated from his father, from his grand-father that :They were with the Prophet (S) on a journey. They wound up in a narrow area when Salat became due. Then it began raining from the sky above them, and it was wet beneath them. So Allah's Messenger (S) called the Adhan while he was on his mount, and then the Iqamah, going forward on his mount. He let them in Salat by making gestures, making his prostrations lower than his bowing