Srimad-Bhagavatam: Canto 6 - Chapter 1 - Verse 4
यमदूता ऊचु:कति सन्तीह शास्तारो जीवलोकस्य वै प्रभो ।त्रैविध्यं कुर्वत: कर्म फलाभिव्यक्तिहेतव: ॥ ४ ॥
yamadūtā ūcuḥkati santīha śāstārojīva-lokasya vai prabhotrai-vidhyaṁ kurvataḥ karmaphalābhivyakti-hetavaḥ
The Yamadūtas said: Our dear lord, how many controllers or rulers are there in this material world? How many causes are responsible for manifesting the various results of activities performed under the three modes of material nature [sattva-guṇa, rajo-guṇa and tamo-guṇa]?
Śrīla Viśvanātha Cakravartī Ṭhākura says that the Yamadūtas, the order carriers of Yamarāja, were so disappointed that they asked their master, almost in great anger, whether there were many masters other than him. Furthermore, because the Yamadūtas had been defeated and their master could not protect them, they were inclined to say that there was no need to serve such a master. If a servant cannot carry out the orders of his master without being defeated, what is the use of serving such a powerless master?