Is it forbidden to put cloth on one's mouth while praying?

I read in Muwatta' Malik, book of The Times of Prayer, Hadith no. 31 or 32 : > وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ عَبْدِ الرَّحْمَنِ بْنِ الْمُجَبَّرِ، أَنَّهُ كَانَ يَرَى سَالِمَ بْنَ عَبْدِ اللَّهِ إِذَا رَأَى الإِنْسَانَ يُغَطِّي فَاهُ وَهُوَ يُصَلِّي جَبَذَ الثَّوْبَ عَنْ فِيهِ جَبْذًا شَدِيدًا حَتَّى يَنْزِعَهُ عَنْ فِيهِ ‏ > > Yahya related to me from Malik from Abd ar-Rahman ibn al-Mujabbar that he used to see Salim ibn Abdullah pull the cloth away fiercely from the mouth of any man he saw covering his mouth while praying. Why did he remove the cloth from them? Is it because this is forbidden? For your information, Salim ibn Abdullah (may Allah have mercy on him) is the grandson of Umar bin Al-Khattab (may Allah be pleased with him). Jazakum Allah khayran