It is narrated from Umm al-Mu'mineen Sayyidah Aisha (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) would recite (Qur'an) while sitting, and when he intended to bow (for ruku'), he would stand for such a duration before bowing as it takes a person to recite forty verses.
Hadith Referenceالسنن المأثورة / ما جاء في الجمع بين الصلاتين في المطر / 29
Hadith Takhrijصحیح مسلم، المساجد ومواضع الصلاة باب جواز النافلة قائما وقاعدًا، رقم: 731