(a) It is narrated from Amr bin Uthman bin Ya'la that the Prophet (peace and blessings be upon him) and his Companions (may Allah be pleased with them) passed through a narrow pass; rain was falling from above and there was wetness below (the narrator is unsure whether the word was 'al-sama'' or 'al-billah'). The time of prayer came; he (peace and blessings be upon him) ordered the mu'adhdhin, and he called the iqamah. Then the Messenger of Allah (peace and blessings be upon him) came forward on his mount and led them in prayer. He (peace and blessings be upon him) was praying by gestures, making the prostrations lower than the bowing. In the narration of Abu Abdullah, Yahya (may Allah have mercy on him) says that perhaps he said: The enemy was above them and the wetness below them.
(b) Its chain is weak; some of its narrators are not reliable, because of which their hadith is not authentic.
It is also possible that this (manner of prayer) was due to the severity of fear.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2224