Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: Sayyiduna Umar bin Khattab (may Allah be pleased with him) said: "Kissing is a part of touching (as mentioned in the Qur'an), so perform ablution after it." This narration is authentic.
Hadith Referenceسنن الدارقطني / كتاب الطهارة / 517
Hadith Takhrij«أخرجه الحاكم فى ((مستدركه)) برقم: 470، والبيهقي فى((سننه الكبير)) برقم: 610، والدارقطني فى ((سننه)) برقم: 517»