Valmiki Ramayana Ayodhya Kanda Sarga 8 Shloka 11
Original Shlokaएवं चेत्त्वं सहास्माभिस्तस्याः प्रेष्या भविष्यसि। पुत्रश्च तव रामस्य प्रेष्यभावं गमिष्यति।।2.8.11।।
1 source checked for this shloka
एवम् in this way, त्वम् you, अस्माभि: सह along with us, तस्या: for her, प्रेष्या an attendant, भविष्यसि if you become, तव your, पुत्रश्च son also, रामस्य to Rama, प्रेष्यभावम् slavishness, गमिष्यति will attain.
If you thus become her attendant along with us, your son will also develop slavishness towards Rama.