Garudapurana 3.28.136

3.28.136

yastaṃbhasūtraṃ kalaśe parītya pūjāṃ kṛtvā vidyamāno gṛhe sve /
yadā tadā yamadūtaśca samyak taṃ staṃbhasūtraṃ tasya mukhe nidhāya //

and wrapping the post-thread around a jar, having performed worship while remaining in his own house, / when at that time Yama's messenger placed that very post-thread into his mouth