Sheikh Salman al-Oadah
The phrase “muttafaq `alayh” literally means “agreed upon”.
Among scholars of hadîth, the term is used to indicate those hadîth that are found in both Sahîh al-Bukhârî and Sahîh Muslim as related by a single Companion.
If the hadîth is related in Sahîh al-Bukhârî by a different Companion than the one who relates it in Sahîh Muslim, then the hadîth will not be referred to as “muttafaq `alayh”.
This is the usage that is well known and well-established among scholars of hadîth. Ibn Hajar al-`Asqalânî says: “It is what they (al-Bukhârî and Muslim) agree to place in their respective books from the hadîth of a single Companion.” [al-Nukat `alâ Kitâb Ibn al-Salâh (1/364)]
Among hadîth scholars, al-Jawzaqî uses the term to refer to any hadîth text found in both Sahîh al-Bukhârî and Sahîh Muslim, regardless of whether it is related in both sources from the same Companion. This is how the term is generally employed by scholars of Islamic Law.
Source: Islam Today