Sayyiduna Ibn Umar (may Allah be pleased with them both) narrated: When a person performs ablution and then touches his armpits, he should repeat the ablution.
(b) It is narrated from Ibn Abbas (may Allah be pleased with them both) that he is not required to repeat it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 656