Please try the below way....
<asp:repeater id="Repeater1" runat="server" xmlns:asp="#unknown">
<itemtemplate>
<table style="width: 100%;">
<tr>
<td><asp:label id="Label1" runat="server" text="<%#Eval("ProductName") %>"></asp:label></td>
<td><asp:textbox id="TextBox1" runat="server" text="<%#Eval("Amount") %>"></asp:textbox></td>
</tr>
</table>
</itemtemplate>
</asp:repeater>
protected void Button1_Click(object sender, EventArgs e)
{
var sb = new StringBuilder();
Response.ClearContent();
Response.Buffer = true;
Response.AddHeader("content-disposition", "attachment;filename=Details.xls");
Response.Charset = "";
Response.ContentType = "application/excel";
sb.Append("<table border="1">");
foreach (RepeaterItem item in Repeater1.Items)
{
sb.Append("<tr>");
sb.Append("<td>");
sb.Append(((Label)item.FindControl("Label1")).Text);
sb.Append("<td>");
sb.Append("<td>");
sb.Append(((TextBox)item.FindControl("TextBox1")).Text);
sb.Append("<td>");
sb.Append("</td></td></td></td></tr>");
}
sb.Append("</table>");
Response.Write(sb.ToString());
Response.End();
}