The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, said: “Whoever is forgetful in the last two rak‘ahs of the Dhuhr, ‘Asr, or Isha prayer and does not know whether he is in the third or fourth rak‘ah, then he should refer to his assumption regarding that. If his assumption inclines more strongly toward either one of them, he should act upon it. But if his assumption is equal between both possibilities, he should act upon the greater number (assume it is the fourth rak‘ah), and then make up what he assumes he has missed. For example, if he doubts between the third and fourth and his assumption is balanced between them, he should assume he is in the fourth, perform the tashahhud, give the taslim, then stand and perform one rak‘ah with tashahhud in it, or perform two rak‘ahs while sitting, and perform tashahhud in the second of them.”
Commentary #8