What is the reason for Imam Malik's choice of the tashahhud formula that includes "al-zakiyyat" (the pure ones), and does it carry more reward? And what is the ruling on the prayer of an elderly woman who insists on saying "I bear witness that there is no god but Allah, and that Muhammad is His servant and His messenger," using a disjoined form of the two testimonies?

The Tashahhud (definition) is valid with any formula established from the Prophet, peace and blessings be upon him, and the difference among scholars lies in what is most preferable. Imam Ahmad explicitly stated the permissibility of all formulas, preferring Ibn Mas'ud's Tashahhud because it is the most authentic and firmly established. Al-Shafi'i chose Ibn Abbas's Tashahhud, and Malik chose Umar's Tashahhud. Sheikh al-Islam preferred that the worshipper recite this one time and that one another time.

As for not repeating the word "ashhadu" (I bear witness) in the Tashahhud, it is not obligatory, and the prayer of one who does not repeat it is valid. Ibn Qudamah mentioned that Ahmad said in Abu Dawud's narration: "If he says 'and that Muhammad is His servant and messenger' and does not mention 'ashhadu', I hope it will suffice him." This is indicated by the omission of this word in some narrations.