Is the prayer valid if a follower joins the Imam during the bowing (Ruku)?
Chapter on the Description of Prayer
Al-Mughni
Book of Prayer
Primary text
The prayer is valid, and the follower has attained the rak'ah if they join the Imam during the bowing. This is based on the Prophet's saying: "Whoever catches the bowing has caught the rak'ah," narrated by Abu Dawud. This applies if the follower achieves tranquility in Ruku, or reaches the minimum measure of Ruku before the Imam moves past that measure. The follower is credited with that rak'ah. Otherwise, if the follower is bowing while the Imam is rising, it is not sufficient.
Supporting text
If the follower joins during Ruku such that the Imam is rising as the follower begins to bow, the initial Takbir (Takbirat al-Ihram) must be said while standing erect. If this Takbir is said during the descent into Ruku or partially during the descent, it is invalid as it is stated in the wrong place. However, an exception exists for voluntary prayers (nafilah). A differing opinion from Umar ibn Abd al-Aziz, Hammad ibn Abi Sulayman, states two Takbirs are due. However, the sound position is that a single Takbir suffices, as reported from Zayd ibn Thabit, Ibn Umar, Sa'id ibn al-Musayyab, 'Ata, al-Hasan, Maymun ibn Mihran, al-Nakha'i, al-Hakam, al-Thawri, al-Shafi'i, Malik, and the Companions of Opinion (Ashaab al-Ra'y). Furthermore, if a single obligatory act (Ihram) fulfills two obligations (like Tawaf al-Ziyarah fulfilling Tawaf al-Wada'), then one Takbir suffices for both the opening and the bowing descent.