Narrated Safiya bint Shaiba: `Aisha used to say: "When (the Verse): "They should draw their veils over their necks and bosoms," was revealed, (the ladies) cut their waist sheets at the edges and covered their heads and faces with those cut pieces of cloth." Bukhari

This hadith says that women covered their faces after the verse 24:31 was revealed. & this hadith is also classified as Authentic(sahih). Does this indicate that covering face is mandatory?