Abdur Rahman bin Abi Saeed narrated from his father that the Messenger of Allah (peace be upon him) said: "When any of you yawns, he should place his hand over his mouth, for the devil enters the mouth." Imam Darimi (may Allah have mercy on him) said: That is, he should place his hand over his mouth.
Hadith Referenceسنن دارمي / من كتاب الصللاة / 1420
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده حسن من أجل نعيم بن حماد، [مكتبه الشامله نمبر: 1422]
Hadith Takhrijاس روایت کی سند حسن ہے، لیکن دوسری سند سے حدیث صحیح ہے۔ دیکھئے: [مسلم 2995] ، [أبوداؤد 5026، 5027] ، [أبويعلی 1162] ، [ابن حبان 2360]
Brief Explanation
(In Explanation of Ahadith 1418 to 1420)
In this narration, «فَلْيَشُدَّ» is mentioned; in some manuscripts, «فَلْيَسُدَّ» appears, and in the narrations of Muslim, the words «فَلْيُمْسِكْ» and «فَلْيَكْظِمْ» are used, which mean to restrain or to swallow (hold back), that is, to restrain as much as possible because it is a sign of laziness, indolence, and heaviness. If it becomes too difficult, then one should place his hand over his mouth.
In some narrations, it is explicitly stated: «إِذَاتَثَاءَبَأَحَدُكُمْفِيالصَّلَاةِ.», meaning when any of you yawns during prayer, he should place his hand over his mouth, because through this action, Satan finds an opportunity to enter, cast whisperings (waswasah) in the prayer, and cause forgetfulness.