Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: "Mulamasah" (touching) refers to whatever is short of intercourse. If a man touches his wife's body with desire, then ablution becomes necessary.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators have been declared trustworthy, except that its chain contains a narrator, Hammad bin Abi Sulayman, about whose use as evidence there is disagreement.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطهارة / 1282