Sayyiduna Ali (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) forbade me to recite (the Qur'an) while in ruku' (bowing). He said: "O Ali, the example of the one who does not straighten his back in his prayer is like a pregnant woman who conceives, and when the time of her delivery draws near, she miscarries; so now she is neither pregnant nor has a child."
Imam Abu Ya'la recorded this narration. I say: Part of this narration, concerning the prohibition of reciting during ruku', is reported in the Sahih. Its chain contains a narrator, Musa bin Ubaydah al-Rabadhi, and he is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2731