Shaykh Muḥammad ibn Sāliḥ al-ʿUthaymīn: If crying during the prayer is due to fear of Allah , or because a person recalls some of the promises and admonitions mentioned in the Noble Qurʾān, then this does not invalidate the prayer. However, if the crying results from remembering a personal misfortune that has occurred, or something similar, then this invalidates the prayer.
This is because something external to the prayer has intruded into it. It is obligatory upon the person to restrain himself from this type of crying so that he does not invalidate his prayer. It has been legislated for him not to concern himself with anything unrelated to the prayer. He should not think about other matters, since thinking about matters that are not connected to the prayer greatly diminishes the reward of the prayer.
References
Ibn al-ʿUthaymīn, Muḥammad ibn Ṣāliḥ (d. 1421 AH / 2001 CE). al-Fatāwā Nūr ʿalā al-Darb. Riyadh: Saudi Broadcasting Corporation, n.d. Vol. 9, p. 141.
