Canto 2 -
Ayodhyā-kāṇḍa
Chapter 107: Rāma Explains why the King Gave the Kingdom to Kaikeyī
Text 2.107.3

पुरा भ्रातः पिता नः स मातरं ते समुद्वहन्।
मातामहे समाश्रौषीद्राज्यशुल्कमनुत्तमम्॥

purā bhrātaḥ pitā naḥ sa mātaraṁ te samudvahan
mātāmahe samāśrauṣīd rājya-śulkam anuttamam

purā = when; bhrātaḥ = brother; pitā = father; naḥ = Our; saḥ = he; mātaram = mother; te = Your; samudvahan = married; mātāmahe = Your maternal grandfather that; samāśrauṣīt = promised; rājya-śulkam = he would pay the price of a kingdom; anuttamam = excellent.

Brother, when Our father married Your mother, he promised Your maternal grandfather that he would pay the excellent price of a kingdom.

Bharata had criticized Their father by stating that he had [given away the kingdom] due to lust and delusion. Here Rāma presents an unassailable argument that this had nothing to do with delusion[on the part of King Daśaratha].

Samāśrauṣīd rājya-śulkam: Daśaratha had promised Kaikeyī’s father that he would give the kingdom to the son born to Kaikeyī.