Valmiki Ramayana Ayodhya Kanda Sarga 28 Shloka 13
Original Shlokaउपवासश्च कर्तव्यो यथा प्राणेन मैथिलि। जटाभारश्च कर्तव्यो वल्कलाम्बरधारिणा।।2.28.13।।
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मैथिलि O Daughter of Mithila, यथाप्राणेन according to one's ability, उपवासश्च fasting also, कर्तव्यः has to be undertaken, वल्कलाम्बरधारिणा wearing robes of bark, जटाभारश्च also burden of matted hair, कर्तव्यः has to be borne.
One has to wear robes of bark and matted hair and take to fasting as long as one can, O daughter of Mithila