Is it permissible for a worshipper who joined the congregational prayer, having missed a part of it, to rely on the person beside him to determine what he missed, even deliberately prolonging his prayer while waiting for him, and not perform the prostrations of forgetfulness (sujood as-sahw) after completing the prayer?

A latecomer to congregational prayer (masbuq), if he doubts how many cycles (rak'ahs) he has missed and no number seems more probable to him, should base his action on certainty. He should make up the greater number that he might have missed, then perform the prostrations of forgetfulness (sujud al-sahw) after the salutation (taslim). It is permissible for him to rely on another latecomer who joined with him if he believes him to be truthful. However, intentionally prolonging the postures (arkan) of prayer does not invalidate the prayer.