Musnad Abu Ya'la al-Mawsili 3:55

Ubayd Allah ibn Umar narrated to us, Asbat ibn Muhammad narrated to us, Musa ibn Ubaydah al-Rabadhi narrated to us, from Abdullah ibn Hunayn, from his father, who said—and his father was one of the scribes of Ali—from Ali, who said: The Messenger of Allah (may Allah bless him and grant him peace) forbade me from reciting while I am bowing. And he said: "O Ali, the likeness of the one who does not straighten his back in his prayer is like the likeness of a pregnant woman who carried [a child], and when her time of delivery drew near, she miscarried; so she is neither one who possesses a pregnancy nor one who possesses a child."