The line Sai said is from Adi Shankaracharya’s Devi Aparadha kshamapana stotram.
विधेरज्ञानेन द्रविणविरहेणालसतया
विधेयाशक्यत्वात्तव चरणयोर्या च्युतिरभूत् ।
तदेतत् क्षन्तव्यं जननि सकलोद्धारिणि शिवे
कुपुत्रो जायेत क्वचिदपि कुमाता न भवति ॥२॥
(O Mother.! Due to Ignorance of the Vidhis (Injunctions of Worship), and due to lack of Wealth, as well as due to my Indolent (Lazy) nature, ...
It was not possible for me to serve Your Lotus Feet; there have been Failures on the performance of my duties (I admit that),
All these are pardonable (by You), O Mother; because You are the Saviour of All, O Shivaa (Auspicious Mother),
There can be Kuputra (fallen disobedient son turning away from Mother), but there can never be Kumata (Mother turning away from son permanently.
Adi Shankara here is asking forgiveness to Devi Maa for the mistakes in performing his duties.
He says however bad the son is,mother will always forgive and never be angry with him forever.